- move accZ computation into imu main function

git-svn-id: http://multiwii.googlecode.com/svn/trunk/MultiWii_shared@1614 02679b44-d973-9852-f2fa-63770883b36c
- reformat accMag condition

git-svn-id: http://multiwii.googlecode.com/svn/trunk/MultiWii_shared@1615 02679b44-d973-9852-f2fa-63770883b36c